The present invention relates to a new and improved construction of a thermostatic expansible work element--also sometimes referred to in the art as a thermostatic expansible material-working element--, which is of the type comprising an expansible body formed of plastic and enclosed in a housing and equipped with a work pin or the like which is pointed at one end, the work pin displaceably extending through an opening in the housing and with its pointed end protruding into a bore in the expansible body. The work pin, during thermal expansion of the expansible body, being forced out of the latter.
Such type thermostatic expansible work element has been disclosed for instance in the German patent publication 2,461,041 and the corresponding U.S. Pat. No. 3,968,692, granted July 13, 1976. With this heretofore known and competent thermostatic expansible work element the expansible body is formed for instance of polytetrafluoroethylene (PTFE), i.e., from a basically solid plastic which does not transform into the liquid phase even at the operating temperatures at which the work element should respond. Furthermore, the thermal expansion of the expansible body enclosed in the housing produces such a compressive load that the expansible body behaves in a plastic manner and, owing to the absence of any other expansion possibilities, forces the work pin out of the expansible body.
Of course this plastic behavior of the expansible body does not proceed to such an extent that such would flow out of the annular gap between the work pin and the inner wall of the opening in the housing, although here, of necessity, thee must be provided a certain play which is capable of permitting the displacement movement of the work pin. With sufficient length of the opening of the housing it is even possible to have the work pin extend without any seal through the opening.
The apparent advantages of the heretofore known thermostatic expansible work element can only be completely realized in practice if the bore provided in the expansible body, into which there extends the pointed end of the work pin, is exactly centrally arranged and furthermore is exactly aligned with the opening in the housing. If this is not so then the work pin, during the expansion of the expansible body, not only will be axially displaced, but also is exposed to a radial force, with the result that the work pin tends to bind or clamp in the opening. The result of this phenomenon is that the temperature-displacement characteristic curve of the thermostatic expansible work element is at least falsified in that the stroke or displacement of the work pin within the response region of the thermostatic expansible work element is not always the same for the same temperature changes.